1. Field of Invention
This invention relates to a method and apparatus for high voltage testing of large electrical coils and in particular to testing turbine generator stator coils with grounded insulation and having end turns with ungrounded insulation at which flash over is likely to occur before the insulation fails.
2. Background Information
Large turbine generators typically have a number of coils inserted in slots in a stator frame. The insulation on the coils is painted with conductive paint to ground the surface of the insulation to the stator frame. Insulated end turns on the coils terminate in uninsulated terminals. Samples of such coils are tested by increasing the voltage applied to the terminals at a controlled rate until the insulation on the coils fails. The usual mode of failure is a puncture in the insulation. Very often, however, flashover occurs before the insulation fails. By flashover it is meant that an arc is struck along the surface of the ungrounded insulation on an end turn between the terminal and the grounded insulation on the coil. The primary cause of flashover is breakdown, or ionization, of the ambient air adjacent the end turn. Humid air breaks down more readily than dry air so that flashover is more likely to occur in a humid atmosphere.
To avoid flashover, so that a true test of the insulation can be made, insulated test bars (laboratory specimens) are immersed in transformer oil or placed in a box filled with silicon hexaflouride, SF.sub.6 gas, both of which have higher breakdown strength than ambient air. While satisfactory for testing small samples, these remedies are much more difficult to apply to full size coils which typically extend 20 feet in length. Huge containers of oil and very large volumes of SF.sub.6 would be required to immerse these full size coils for testing. In addition, the large volumes of oil and gas are difficult to handle and would require large pieces of equipment. Furthermore, the oil is difficult to keep clean and impurities can alter its dielectric properties. The oil can also penetrate the insulation which affects the results of the tests.
With this background in mind, it is the primary object of the present invention to provide a method and apparatus for high voltage testing of coils with grounded insulation, ungrounded insulation on the end turns and uninsulated terminals which require a minimum of equipment and fluid, and are economical, and easy to use.